The change, inserted by Rep. Bob Brady (D-Pa.), chairman of the committee charged with handling the bill and a key union ally, would also affect other groups funded by members who pay dues of less than $50,000. While the move may satiate liberal Democrats who had become uneasy with an exemption for the National Rifle Association, a union loophole will certainly cause big business to cry foul.

The bill could come up for a vote Thursday on the House floor.

The underlying bill is intended to blunt the impact of a January Supreme Court ruling overturning laws that had barred corporations and unions from airing certain types of election ads.

Democrats feared the ruling would pave the way for vast corporate-funded ad campaigns attacking them in the critical 2010 midterm elections, and Republicans have blasted the DISCLOSE Act as a Democratic effort to silence opponents. . . .